Cancer
Cancer accounts for nearly one-quarter of deaths in the United States, exceeded only by heart disease. In the year 2000, there were 553,091 cancer deaths in the US. In 2003, the American Cancer Society estimates that this number will increase to approximately 556,500, due to aging and growth of the population. Lung cancer is the most common fatal cancer in men (31%), followed by prostate (10%), and colon & rectum (10%). In women, lung (25%), breast (15%), and colon & rectum (11%) are the leading sites of cancer death. Among children, leukemia is the most common cancer among children ages 0-14 years and it comprises approximately 30% of all childhood cancers and accounts for the most childhood deaths. Acute lymphocytic cancer is the most common form of leukemia in children. It is estimated that 1.33 million new cases of cancer were diagnosed in 2003 (American Cancer Society, 2003 Cancer Statistics Slide Set 2003).
Melanoma. Studies have indicated that nearly 45,000 new cases of melanoma are diagnosed each year in the U.S. and approximately 20% of patients will die of metastatic disease. Melanomas arise from the malignant conversion of melanocytes, which in turn are derived from mesenchymal neural crest cells. Melanomas undergo melanogenesis, a complex process that results in the production of melanin. Melanogenesis is initiated by the hydroxylation of L-tyrosine, to form L-dihydroxyphenylalanine (L-DOPA), which is then converted to DOPAchrome by specific melanocyte-associated enzymes, including tyrosinase. A further series of oxidation and reduction reactions ultimately convert DOPAchrome to melanin. It has been suggested that melanogenesis may account for the resistance of melanomas to treatment with ionizing radiation and chemotherapy. It has also been suggested that byproducts of melanogenesis are responsible for other adverse effects including immunosuppression, fibrosis and mutagenesis.
Current Treatments
Chemotherapy (CT) and radiation therapy (RT), and combinations thereof, remain the leading defenses against cancer, although recent advances in the field have led to widespread uses of specialized treatments such as angiogenesis inhibitors, biological therapies, including adjuvant therapy to boost the patient's immune system, antibody therapy, vaccine therapy, and photodynamic therapy.
In addition to numerous adverse effects of RT and CT, a major limiting factor is the development of drug resistance by the tumors, and induction of tumor cell growth arrest and senescence. While senescent tumors do not increase in size per se, they still retain the capacity to produce and secrete tumor stimulating mitogens and pro-angiogenic factors that can lead to tumor progression.
The present inventors have previously unexpectedly shown that PABA, which inhibits tyrosinase and melanogenesis, in addition to being a therapeutic for melanotic tumors, can also potentiate treatment of non-melanotic carcinomas with RT and CT (see co-pending U.S. Provisional Application Ser. No. 60/538,359 filed Jan. 21, 2004. Accordingly, it was hypothesized that benzoic acid derivatives that also inhibit tyrosinase may also have this activity for both melanotic and non-melanotic cancer.
Cell Cycle Regulation and PXR/SXR Receptors
Induction of cell-cycle arrest and by activation of G1/S and G2/M check points is known to be critical for the capacity of the cell to undergo DNA repair prior to re-entering the cell-cycle. Moreover, studies have provided that overriding the cell cycle check point controls in cells with DNA damage (e.g., induced by RT or CT), can force the cell to re-enter the cell cycle without repairing the damage, leading to mitotic catastrophe and cell death by apoptosis.
Recent studies have shown that parameters such as the degree of oxygenation/hypoxia, the expression and function of DNA repair proteins, cell cycle and checkpoint control proteins, and cell adhesion and extracellular matrix proteins all regulate tumor radiosensitivity. In particular, in response to DNA-damaging agents such as RT and CT, several kinases are activated (e.g., ATM and ART) which phosphorylate check point control proteins such as Chk2, which in turn, phosphorylates CDC25A, targeting CDC25A for degradation. Degradation of CDC25A leads to cell cycle arrest and DNA repair, while overexpression of CDC25A has been demonstrated to activate Cdk2 kinase, leading to accumulation of Cyclin E/Cck2 complexes and cell cycle progression followed shortly by cell death. Studies have suggested that alteration of cell cycle regulators such as CDC25A, CHK-1 and CHK-2 can sensitize sells to death by RT and CT, suggesting that agents that inhibit cell cycle arrest or promote cell cycle transition may be useful as adjuvant therapy for RT or CT.
Induction of cell cycle DNA repair proteins in response to cell cycle arrest is also hypothesized to have a role in radio- and chemosensitivity. Studies have shown that BRCA-2, a DNA repair protein, can regulate transcription of RNA polymerase II, or directly bind to Replication Protein A to regulate DNA repair. Accordingly agents that functionally inactivate or decrease expression of BRCA-2 may enhance cell cycle progression, and hence, enhance sensitivity of tumor cells to CT and RT (Wong et al., Oncogene 2003; 22: 28-33; and Kraakman-van der Zwet et al., Mol. Cell Biol. 2002; 22: 669-679).
para-amino-benzoic acid (hereinafter “PABA”), is a water-soluble naturally-occurring compound that is essential for microorganisms and some animals, but not humans. PABA also has been shown to inhibit cell cycle arrest and DNA repair. Studies in Xenopus embryos have revealed a signaling pathway mediated by endogenous benzoic acid derivatives (i.e., PABA derivatives such as 3-hydroy ethyl benzoate or 3-HEB) and orphan nuclear hormone receptors, benzoate X receptors (BXR). BXR heterodimerizes with the orphan retinoic acid X receptors (RXR) to regulate gene expression. A human and rodent homologue of BXR has been identified and is designated Pregnane X receptor or steroid X receptor (PXR/SXR). PXR/SXR is a member of xenobiotic nuclear receptors which function in induction of cytochrome P450 enzymes involved in drug metabolism and detoxification of xenobiotic compounds, and have limited expression restricted to the liver and small intestine. PABA and PABA derivatives have been shown to bind and activate the PXR/SXR receptor (Moore et al., Mol. Endocrinol. 2002; 16: 977-986).
Interestingly, the PXR/SXR receptor has also been found to be abnormally expressed in some tumors, such as melanoma (see Moore, supra). The present invention demonstrates that PABA and PABA derivatives which bind to PXR/SXR can surprisingly potentiate tumoricidal effects of CT and RT in tumor cells overexpressing PXR/SXR, likely by inhibiting cell cycle arrest and DNA repair through transcriptional regulation of cell cycle regulatory proteins.
There remains a need in the art for therapies that inhibit cell cycle arrest and DNA repair, and hence, cell senescence specifically in tumor cells, in order to sensitize tumor cells to killing by RT and CT. Accordingly, it is likely that specific PXR/SXR agonists may provide a novel approach to enhance the anti-tumor effects of CT and RT in tumor cells and not normal cells.